Photoconductive ternary alloy of cadmium,selenium and mercury

ABSTRACT

A NOVEL PHOTOCONDUCTIVE CRYSTALLINE TERNARY ALLOW PREPARED FROM A HOST MATERIAL CONTAINING CADMIUM, SELENIUM AND MERCURY AND DOPED WITH AN ACTIVATOR AND COACTIVATOR, SAID HOST MATERIAL BEING FIRED AT LEAST ONCE UNTIL THE CRYSTALLIZATION THEREOF IS COMPLETE AND A PHOTOSENSITIVITY PEAKING UP IN THE INFRARED SPECTRAL RANGE IS ATTAINED. THE ACTIVATOR IS SELECTED FROM ANY OF IB GROUP ELEMENTS CONSISTING OF COPPER AND SILVER AND THE COACTIVATOR IS SELECTED FROM ANY OF VIIB GROUP ELEMENTS CONSISTING OF CHLORINE, BROMINE AND IODINE OR FROM ANY OF IIIB GROUP ELEMENTS CONSISTING OF ALUMINUM, GALLIUM INDIUM.

y 1972 TADAO NAKAMURA 3,665,45Q

PHOTOCONDUCTIVE TERNARY ALLOY OF CADMIUM, SELENIUM AND MERCURY Filed March 10, 1970 5O/\\ \IQ O8 O9 IO 1.! I2 [.3 [-4 l5 WAVELENGTH MICRONS NORMAL! ZED PHOTOCURRENT IVEN QRS Tags! nRamuR l EJ11 515 n\\nmu.%% TRAHU m5 ATT 3,660,450 Patented May 30, 1972 rm. (:1. CZZc 7/00, 17/00 US. Cl. 75-134 H 8 Claims ABSTRACT OF THE DISCLOSURE A novel photoconductive crystalline ternary alloy prepared from a host material containing cadmium, selenium and mercury and doped with an activator and coactivator, said host material being fired at least once until the crystallization thereof is complete and a photosensitivity peaking up in the infrared spectral range is attained. The activator is selected from any of Ib group elements consisting of copper and silver, and the coactivator is selected from an of VIIb group elements consisting of chlorine, bromine and iodine or from any of I IIb group elements consisting of aluminum, gallium and indium.

The present invention relates to a novel photoconductive crystalline ternary alloy containing cadmium, selenium and mercury and having photosensitivity which peaks up in the infrared spectral range. The invention includes improved methods for preparing the photoconductive alloy.

A photoconductive device is one which displays a reduced resistance to an electric current flow when irradiated, as for example, vw'th light. In its simplest form, a photoconductive device comprises a body of photoconductive material and a pair of electrodes attached thereto. With a voltage applied to the electrodes, the device passes an increased amount of electric current in the event of an increase in the intensity of light irradiating the device.

The photoconductive alloy of the invention as applicable for such device is based on the discovery made by us that the ternary alloy containing cadmium, selenium and mercury behaves as a semiconductor of N type. This alloy exhibits photoconductivity particularly to irradiating light of the infrared range, when doped with suitable impurities. The semiconductor of the invention is composed substantially of ternary alloy doped with lb and VIIb group elements as impurities, which alloy is expressed, in the general form, Cd Hg Se, where x l. With respect to the doped impurities, the Ib group element acts as an activator while the VIIb group element as a coactivator. IIIb group elements may also be employed in place of the VIIb group elements.

In the general expression Cd Hg Se of the photoconductive alloy of the invention, x may assume any value so long as it lies in the range between zero and one. The alloy is rendered into a usual compound of H-gSe having no photoconductivity for x=0 and a conventional photoconductive material of CdSe for x: 1. Therefore, x should be larger than zero but smaller than 1 for practical applications of the alloy if it is utilized in a photoconductive device sensitive to the infrared radiation. Experiments thus far conducted have revealed that as x approaches zero the spectral sensitivity of the alloy shifts toward the longer wavelength in the infrared range.

Some of known methods for producing the existing photoconductive materials can be adopted to produce the novel ternary alloy of the invention. Difiiculty is, however, experienced in the conventional methods of producing, such alloy, because the pressure of mercury vapour used is excessively high. For example, mercury will evaporate readily at an elevated temperature of about 1,000" C. even though it is placed under a high pressure of 100 atm., for instance. This evaporation can not be prevented in the conventional methods which include, as is well known, at least one firing step partly for promoting crystallization of the host material and partly for introducing the impurities into the host crystal. Thus, mercury added is not introduced into the host material but evaporates into the surrounding atmosphere during the firing process. 7

This is reflected by the fact that an alloy with a desired amount of mercury introduced thereinto can not be prepared by such existing methods.

It is therefore a primary object of the present invention to provide a novel photoconductive crystalline ternary alloy prepared from a host material containing cadmium, selenium and mercury and doped with activator and c0- activator impurities.

It is another object of the invention to provide a photoconductive alloy provided with a photosensitivity peaking up in the infrared spectral range.

It is a further object of the invention to provide improved methods for preparing the photoconductive alloy of the invention.

Photoconductive crystalline ternary alloy according to the invention is prepared by firing the host material containing cadmium, selenium and mercury and by doping the host material with vIb group elements including copper and silver as an activator and VIIb or IIIb group elements including chlorine, bromine and iodine, or aluminum, gallium and indium, respectively, as a coactivator. The photoconductive alloy of the invention is characterized in that the alloy exhibits its maximum photosensitivity in the infrared spectral range. A photoconductive device according to the invention comprises a mass of the photoconductive alloy of the invention with or without a binder and a pair of electrodes attached thereto.

I The invention will be more fully described in the following 1n conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view showing a photoconductive device according to the invention; and

FIG. 2 is a series of characteristic curves showing the spectral sensitivities of some typical photoconductive alloys of the invention.

In order to prevent mercury or one of the host substances from evaporating during the firing process, there are proposed by the invention some methods for preparing the photoconductive alloy.

In one preferred method of the invention, the host material containing cadmium, selenium and mercury each in the form of elementary substance is fired twice at two different temperatures. More specifically, a starting or host mixture contains predetermined amounts of pure cadmium, selenium and mercury. This starting mixture is fired preferably at 700 to 800 C. under the pressure of to atm. in an inert gas atmosphere until a homogeneous a1- loy is obtained therefrom. The product obtained after the first firing is re-fired at 1,100 to l,200 C. under the pressure of 70 to 100 atm. in an inert gas atmosphere until the crystallization of the starting material is complete. In this instance, the introduction of the impurities can be performed in two different fashions. For example, the impurities to be added are previously mixed with the starting material and therefore the succeeding firing and re-firing are performed in the presence of the impurities to be doped. As a result, the starting material is doped with the impurities during the two firing processes. The photoconductive alloy obtained in this fashion is in the form of polycrystal. In the other fashion, however, the impurities are introduced into the crystallized alloy by the method such as thermal diffusion method upon completion of the,

firing processes.

The host material being fired at least twice at different temperatures, this method can prevent the mercury from evaporating into the surrounding atmosphere. before'it is introduced into a finished ternary alloy. This is because the mercury once introduced into the ternary alloy Will not leave there and evaporate into the atmosphere.

In another preferred method proposed by the invention mercury is added in the form of its compound to the starting material containing cadmium and selenium in the form of cadmium selenide, for instance. It should be appreciated as a feature of this method that a solvent flux is further added to the starting material in order to reduce the melting point of the material and therefore to accompli'sh the crystal growth of the material at a reduced temperatur'e. In this method, the activator impurity is added to and mixed with the starting material while the coactivator impurity is supplied from the solvent flux. In this particular respect, the mercury compound may act as the solvent flux and as a supplier of the coactivator impurity. According to this method, the amount of the mercury added in the form of its compound to the starting material before the -firing process does not correspond strictly to that of the mercury contained in the finished photoconductive alloy. This is because of the fact that predetermined amount of the mercury added cannot be restrained from evaporating into the ambient atmosphere during the firing process. Consequently, this method is advantageously applied for preparing the photoconductive alloy with decreased content of mercury, namely, with its photosensitivity to the infrared spectral range relatively near the visible light.

Referring now to FIG. 1, there is shown a photoconductive device using the photoconductive alloy of the invention. The photoconductive device, which is designated in general as numeral 10, comprises customarily the photoconductive ternary alloy 11 of the form Cd Hg Se, a pair of electrodes 12 mounted on the alloy and spaced from each other and a pair of lead wires 13 attached to the respective electrodes 12. The electrodes 12 are, 1n lIhIS instance, evaporated onto the surface of the photoconductive alloy 11. In operation, a suitable voltage of either DC. or AC. type is applied between the two electrodes 12 to produce a photocurrent when irradiated by the infrared ray. This photocurrent is measured to inspect the spectral photosensitivity of the device 10.

In FIG. 2 showing a series of the characteristic curves of some typical photoconductive alloys to be employed in the device of FIG. 1, the abscissa represents the wavelength of the infrared ray incident on the photoconductive device Whereas the ordinate represents normalized photocurrents of the respective alloys as applicable in the device. Detailed discussion of FIG. 2 will be given in: the following in connection with the examples, which are given in order that the present invention be more fully understood.

EXAMPLE 1 The starting material contains g. of cadmium s'el'enid'e and mercury selenide powders of 99.999% purity, 20cc. of 0.001 mole copper salt and 0.5 g. of cadmium halide. The copper salt as applicable in this example is chloride, bromide, iodide or sulphate of copper while the cadmium halide may be any of chloride, bromide and iodide of cadmium. The process used for preparing the photoconductive crystalline ternary alloy comprises the steps of mixing homogeneously the starting material with each other at a temperature of 90 to 100 C. by sutficiently agitating; drying the obtained mixture at aboutl20 C. for about 17 hours in an atmosphere of a circulating air; firing the dried mixture, which is placed on a quartz boat, at about 550 C. for about 4 hours in an atmosphere of a circulating air; and passing the fired mixture through a sieve to remove lumps therefrom.

In this example, the activator impurity is copper supplied from the copper salt and the coactivator impurity is halogen supplied from the cadmium halide acting as a solvent flux. The amount of the mercury added to the starting material does not correspond precisely to that of the mercury contained in the finished photoconductive alloy. More specifically, the mercury content in the finished alloy is less than that added at the mixing step. According to the process described in this example, therefore, the finished alloy exhibits photosensitivity in the nearinfrared range. This can be ascertained from observation of FIG. 2.

As shown in FIG. 2, curve 14 stands for the normalized photocurrent of the conventional photoconductive material of cadmium selenide type, which corresponds to Cd Hg Se for x=1. In the case of the conventional photoconductive alloy, it is clearly understood that the maximum value of the photocurrent is, from the curve 14 in FIG. 2, below 0.8 micron in wavelength. In contrast, curves 15 and 16 have their maximum values at 1.0 and 1.05 micron wavelength, respectively. The curves 15 and 16 are obtained from the photoconductive alloys of the invention, containing 25% and 50% by weight of mercury selenide, respectively, in the starting material.

Comparison of these curves 14, 15 and 16 with each other will readily reveal that photoconductive alloys according to the present invention advantageously exhibit a photosensitivity in the near-infrared range. It is also observed that the spectral sensitivity thereof can provide a relatively wide choice if the mercury selenide content in the starting material is changed.

EXAMPLE II The starting material contains 10 g. of cadmium selenide powders of 99.999% purity, cc. of 0.001 mole copper salt and 4 g. of mercury bromide. The copper salt as applicable in this example is chloride, bromide, iodide or sulphate of copper and mercury bromide acting as a solvent flux may be replaced with either of chloride and iodide of mercury. The process used for preparing the photoconductive crystalline ternary alloy comprises the steps of mixing homogeneously the starting material with each other at an elevated temperature by sufliciently agitating; drying the resultant mixture at about C. for 17 hours in an atmosphere of a circulating air; firing the dried mixture at'about 500 C. for about 5 hours in an atmosphere of a circulating air; passing the fired mixture through a sieve to remove lumps therefrom; re-firing the sieved mixture at about 480 C. for about 10 minutes in an evaporated selenium atmosphere; and further refiring the mixture at about 480 C. for about 15 minutes in a vacuum to remove the residual selenium therefrom by evaporation.

In this example, the activator impurity is copper supplied from the copper salt and the coactivator impurity is bromine supplied from the mercury bromide. The evaporated selenium atmosphere used in the second firing step may be replaced by an evaporated sulphur atmosphere with similar results. The amount of the mercury added to the starting material does not correspond to that of the mercury contained in the completed photoconductive alloy. According to the process of this example, the alloy obtained possesses a maximum photosensitivity in the near-infrared range, namely, at 1.15 micron wavelength, as indicated by curve 17 of FIG. 2.

The photoconductive ternary alloy of the invention may'be utilized for photoconductive devices and elements for detecting the incident infrared rays. These devices are useful in instrumental meters, relays, image converters, pickup devices, switches and so forth. The deviecs comprise customarily the photoconductive alloys of the invention and a pair of electrodes attached to the surface of the alloys. Where the alloys are in the form of polycrystal, suitable resinscan be employed to bond the polycrystalswith each other.

There have been described hereinbefore novel photoconductive ternary alloys containing cadmium, selenium and mercury and doped with an activator and coactivator. The alloys exhibit maximum photosensitivities in the near-infrared range. There have been also described novel methods for preparing the photoconductive alloys of the invention and photoconductive devices comprising the photoconductive alloys. These methods are adapted to prevent the mercury to be added to the photoconductive alloys from evaporating into the surrounding atmosphere during the firing step. The methods can also shift the maximum photosensitivity within a relatively wide range by changing the mercury content in the photosensitive alloys of the invention.

What is claimed is:

1. Photoconductive crystalline alloy comprising cadmium, mercury and selenium, said alloy having the general formula:

wherein the value of x is in the range of x 1.

2. Photoconductive crystalline alloy according to claim 1, further comprising an activator selected from the group consisting of copper and silver, and a coactivator selected from the group consisting of chlorine, bromine, iodine, aluminium, gallium and indium.

3. A method for preparing the photoconductive crys talline alloy of claim 1, comprising the steps of: mixing cadmium, mercury and selenium with an activator selected from the group consisting of copper and silver, and a coactivator selected from the group consisting of chlorine, bromine, iodine, aluminium, gallium and indium, and then firing the mixture at a temperature of from about 450 to about 600 C. under atmospheric pressure in air.

4. A method for preparing the photoconductive alloy of claim 1, comprising the steps of: mixing cadmium, mercury and selenium with an activator selected from the group consisting of copper and silver, and a coactivator selected from the group consisting of chlorine, bromine and iodine, aluminium, gallium and indium; and then firing the mixture at a temperature of from about 700 to about 800" C. under a pressure of from about 70 to about 100 atm. in an inert gas atmosphere.

5. A method for preparing a photoconductive crystalline alloy of claim 1, comprising the step of: mixing cadmium, mercury and selenium; firing the mixture at a temperature of from about 450 to about 600 C. in an atmospheric pressure until a homogeneous material is obtained; re-firing the resultant material at a temperature of from about 450 to about 600 C. in atmospheric pressure until the material crystallizes, and then doping the obtained material with an activator selected from the group consisting of copper and silver, and a coactivator selected from the group consisting of chlorine, bromine, iodine, aluminium, gallium and indium.

6. A method for preparing a photoconductive alloy of claim 1, comprising the step of: mixing cadmium, mercury and selenium; firing the mixture at a temperature of from about 700 to about 800 C. under the pressure of from about 1,100 to about l,200 C. under a pressure of from about to about 100 atm. in an inert gas atmosphere until the material crystallizes, and then doping the obtained material with an activator selected from the group consisting of copper and silver, and a coactivator selected from the group consisting of chlorine, bromine, iodine, aluminium, gallium and indium.

7. A method for preparing a photoconductive alloy of claim 1, comprising the steps of: mixing cadmium selenide, mercury selenide, copper salt selected from the group consisting of chloride, bromide, iodide, and sulphate of copper, and cadmium halide selected from the group consisting of chloride, bromide, and iodide of cadmium; drying the mixture; and then firing at least once the resultant mixture at a temperature above the melting point of cadmium halide in an atmosphere of air.

8. A method for preparing a photoconductive crystalline alloy of claim 1, comprising the steps of: mixingv cadmium selenide, mercury halide selected from the group consisting of chloride, bromide and iodide of mercury, and copper salt selected from the group consisting of chloride, bromide, iodide and sulphate of copper, and then firing at least once the mixture at a temperature above the melting point of mercury halide in an atmosphere of air.

References Cited UNITED STATES PATENTS 2,953,690 9/1960 Lawson et a1. 250-211 3,468,363 9/1969 Parker et al. 164-125 3,514,347 5/1970 Rodot et a1 148186 L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner U.S. Cl. X.R.

UNITED STATES PATENT swim V QETIMQA'EE @f QQREQHN Patent No. 3, 666 450 Dated May 30, 1972 (s) Tadao Nakamura, Shigeaki Nakarrrura, Tadao Kohashi It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, column 5, line 21, the formula reading "0001" should read --O x l-.

Signed and sealed this 6th day of March 1973.

(SEAL) Attest:

EDWARD M.PLETCHER,JR. ROBERT GO TTSCHALK Attesting Officer Commissioner of Patents FORM PO-IOSO (IO-69) USCOMM-DC 60376-P89 fi US4 GOVERNMENT PR NTING OFFICE: 969 0-366-334,

UNITED STATES PATENT oFFicE CERTIFICATE OF COEQTIN Patent No. 3, 666 450 Dated May 30, 1972 fls) Tadao Nakamura, 'Shigeaki Nakamura, Tadao Kohashi It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, column 5, line 21, the formula reading "O x l" should read --O x l-.

Signed and sealed this 6th dayof March 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents ORM PO-IOSO (10-69) USCOMM-DC 6O376-F69 & U45. GOVERNMENT PRINTING OFFICE: I969 0-355'33, 

